The present invention relates to a system for improving flip chip performance. More specifically, the present invention relates to a system and for improving flip chip performance by removing the underfill beneath a flip chip assembly.
Flip chip assemblies are used in many applications today due to advantages in size, flexibility, performance, reliability and cost over other packaging assemblies. Such assemblies provide direct electrical connection of face down, or flipped, electrical components onto substrates, circuit boards, or other carriers using conductive bumps on chip bond pads. Older technologies such as wire bonding use face-up chips having longer lead lengths to input/output (I/O) pads.
Flip chip assemblies provide for substantial reductions in the required board mounting area as flip chip board connections are substantially shorter than those of other packages having contact leads. Flip chip assemblies often give the highest electrical performance of any assembly method. High electrical performance is due to much shorter lead lengths to I/O pads as compared to wire bonding, resulting in reduced capacitance and inductance. Greater I/O flexibility is achieved by use of an entire active surface of the flip chip die for connections rather than only perimeter locations as is the convention for wire bonding techniques.
Flip chip assemblies also feature a rugged interconnection method. With a number of interconnections across the surface area of the die and a commonly used adhesive underfill, the flip chip interconnections to carrier substrates are extremely robust. The adhesive underfill typically joins the entire surface of the flip chip die to a carrier substrate. The underfill protects electrical connections from moisture or other similar hazards while also providing additional mechanical strength to the interconnection between the flip chip and the carrier substrate. More importantly, however, the underfill compensates for any mismatch in the coefficient of thermal expansion (CTE) between the flip chip and the carrier substrate. The underfill is generally comprised of a non-conductive material such as epoxy or other non-conductive adhesives.
In some applications, however, use of an underfill can be undesirable. Interposers, which provide electrical connections between chips of one form factor to carrier substrate footprints of another form factor (e.g., integrated circuit to ball grid array (BGA)), have been proposed to connect flip chip assemblies to carrier substrates. However, such interposers often require an increased footprint as compared to that of the flip chip and use an underfill.